In recent years, fuel cells have been drawing attentions as a clean energy source, and studies for improvement and practical application of the fuel cells have been intensively conducted on power generations for home use, for business use and for automobile use. Among the fuel cells, a solid oxide fuel cell is promising as power sources for home and business uses because of the excellent efficiency and superior long-term stability.
As an electrolyte sheet of a solid oxide fuel cell, a zirconia sintered sheet has been used from the view point of the strengths. With the advancements of practical application studies of a solid oxide fuel cell, mass production of a zirconia sheet has become possible. However, the number of the sheet having defects such as warping, swelling and scratches also has been increased. Such defective sheets cannot be used as the electrolyte sheets, since the durability of a solid oxide fuel cell becomes shorten when the sheets are used. Since zirconia and scandia that are used for the stabilization of zirconia crystal structure are expensive, if the defective sheets are just discarded, the manufacturing costs of solid oxide fuel cells are increased.
In the invention described in Patent Document 1, a zirconia powder is prepared from a zirconia sintered body, then the zirconia powder is re-molded, and a zirconia sintered body is obtained by sintering. In more detail, a zirconia sintered body mainly stabilized by 1.5-5 mol % of yttria is heated to increase the ratio of monoclinic phase to be 50% or more and prepare fine sol particles. The fine sol particles are further milled if necessary. The sol particles are dried and granulated, and then molded, and are sintered again. It is described that the re-sintered body obtained by the said invention possesses physical properties equal to those of usual sintered bodies. The Patent Document 1 discloses a desirable average primary particle diameter and average secondary particle diameter (average diameter of aggregated-particles) of the recycled particles. When the average primary particle diameter exceeds 0.3 μm, extremely high temperatures is necessary to sinter the recycled particles. Thus, it is concluded that the powder should be very fine, 0.3 μm or less.
Also in the inventions described in Patent documents 2 to 5, the average primary particle diameters or average secondary particle diameters of zirconia unsintered powders are specified, although a zirconia sintered powder is not used in the inventions. In the said inventions, the reason why the average primary particle diameters are specified is that when the average primary particle diameter is too small, the necessary amount of binder becomes larger and firing shrinkage of the obtained sheet by sintering becomes large. As a result, dimensional stability is decreased. On the other hand, it is described that when the average primary particle diameter is very large, the strength of the sheet is decreased. The reason of specifying the average secondary particle diameters in the said Patent Documents is for suppressing the defects of the surface of the sintered body and improving the sintering properties.